Orthopedic screws are currently known and used for surgical procedures such as spinal fixation. Accordingly, it follows that fixation of an implant or fixation device is important. Fixation of the implant/fixation device is achieved by securing the implant or fixation device to the spine vis-à-vis the orthopedic screw.
As shown in FIG. 1, previously known orthopedic screws A include an external thread B extending around a screw shaft C. The threads B of the previously known screws A are formed having a solid cross-section with a generally triangular shape. Specifically, the space between the outermost end of the thread or “crest” D (the vertex of the triangular cross-section) and the shaft C of the screw contains some material.
Thus, the cross-section is filled-in with material. In some cases, this material may be a byproduct of the process by which the screw thread is generated. For example, it would be difficult to remove material in a cutting process at such an acute angle. In a rolling process, it is by nature impossible to remove the material. This material may provide strength to resist deformation of the thread or to prevent fracture of the shaft.
The threads B of the previously known orthopedic screws A may become loose after insertion into a patient and begin to exhibit a condition known as “pull out” in which the thread no longer engages enough bony material to provide rigid fixation. This may be due to the bone becoming weak or from the thread deforming in some manner to decrease the outer diameter such as rounding of the crest. As such, the screw may exhibit a decrease in “pull out strength” or the force required to pull the screw along its longitudinal axis and remove it from the bone without unscrewing. As used herein, “pull out strength” refers to the amount of force necessary to remove the orthopedic screw from the insertion site without rotation thereof.
Attempts have been made to improve the pull out strength of orthopedic screws. For instance, orthopedic screws have been manufactured with an expanding thread section. This expanding section may be formed of a mesh or expanding leafs that increases in diameter using a secondary deployment mechanism
However, orthopedic screws with an expanding thread section may also be subject to failure. In some instances, the expanding thread section generates a weak point along the screw shaft. Further, screws with an expanding thread section may also be difficult to revise in a subsequent surgery due to bone ingrowth into the voids in the expanded thread section. The bone ingrowth locks the distal end of the screw in place and can prevent the collapse of the expanded section. When trying to remove an un-collapsed screw bone ingrowth can cause the distal end of the screw to fracture making complete screw removal difficult to achieve. Therefore, revisions to the screw site become problematic.
Thus, there exists a need in the art to improve the previously known orthopedic screws so as to increase a pull out strength while maintaining the ability for revisions to the screw site to facilitate subsequent procedures.